(1) Field of the Invention
The present invention relates to a digital modulation apparatus.
(2) Description of the Related Art
In recent years, with the diversification of communication services and the increase of demands for information communications, high-speed, large capacity and long distance digital transmission has been performed in a basic trunk transmission line using an optical fiber or a CATV transmission line. The improved performance of an optical device and the technological achievement of a high speed for an LSI have greatly reduced costs for an optical transmission device.
Considered as one of radio transmission systems for supporting such an optical transmission foundation is trunk type multiplex radio transmission which uses a microwave band. Radio transmission data provides for an overall framework for frequency assignment, and so on. For achieving a much higher speed for radio digital transmission, a multivalued digital transmission system such as a QAM (Quadrature Amplitude Modulation) or a QPSK (Quadri Phase Shift Keying) has been employed to secure a high bit rate even in a narrow frequency band.
Described below as systems for converting baseband signals into signals of transmission frequencies with reference to FIGS. 16 to 19 are four kinds including an analog system of one kind and digital systems of three kinds.
Referring to FIG. 16, shown are main sections of the transmitter of an analog type radio transmitter-receiver. An analog modulation apparatus 50 shown in FIG. 16 up-converts a baseband modulating signal into a signal of an RF (Radio Frequency) band and then transmits this signal to a radio channel. The analog modulation apparatus 50 comprises roll-off filters 51a and 51b for bang-processing baseband signals to reduce intersymbol interference to a minimum, a frequency conversion unit 54 for frequency-converting the outputs of these roll-off filters 51a and 51b into signals of an RF band and a transmission unit 55 for transmitting the RF signals to the radio channel.
Herein, the frequency conversion unit 54 frequency-converts the outputs of the roll-off filters 51a and 51b and then up-converts the same into signals of an RF band. The frequency conversion unit 54 is constructed to include a D/A (Digital/Analog) converter 52a for converting the output of the roll-off filter 51a from digital to analog and thereby obtaining a multivalued baseband signal, a D/A converter 52b for D/A-converting the output of the roll-off filter 51b, a first frequency conversion unit 53a for up-converting the baseband signal outputted from the D/A converter 52a into a signal of an RF frequency, a second frequency conversion unit 53b for up-converting a baseband signal outputted from the D/A converter 52b into a signal of an RF frequency, a 90xc2x0 phase sifter 53e for inputting the output of a carrier generator 53d to the first frequency conversion unit 53a and an output phase-shifted by 90xc2x0 to the second frequency conversion unit 53b, and a hybrid unit 53c for coupling the outputs of these first and second frequency conversion units 53a and 53b. 
The transmission unit 55 transmits an RF signal outputted from the hybrid unit 53c to the radio channel. The transmission unit 55 is constructed to include a band-pass filter 55a for limiting a transmission band and thereby eliminating unnecessary harmonic components and an antenna 55b for transmission to the radio channel.
In the analog system configured in the above manner, baseband signals are outputted from the roll-off filters 51a and 51b, and then the baseband signals are frequency-converted into signals of an RF band. This analog system is disadvantageous for use in that there is an effect of variance in performance between RF elements and a circuit fine adjustment is necessary. However, with the achievement of a high speed for a device and the improvement of a high-level LSI technology, the analog system is replaced by a digital system, and now it is possible to increase the accuracy of and miniaturize a modem.
Referring to FIG. 17, shown are main sections of the transmitter of a radio transmitter-receiver of a digital modulation system. A digital modulation apparatus 56 shown in FIG. 17 performs quadrature amplitude modulation for an inputted data signal and then transmits the signal to a radio channel. The digital modulation apparatus 56 comprises roll-off filters 51a and 51b, a transmission unit 55 and a quadrature amplitude modulation unit 57.
Herein, the quadrature amplitude modulation unit 57 performs quadrature amplitude modulation for baseband signals outputted respectively from the roll-off filters 51a and 51b. The quadrature amplitude modulation unit 57 is constructed to include a first frequency conversion unit 57a, a second frequency conversion unit 57b, a hybrid unit 57c, a carrier generator 57d, a counter 57e, a cosine information/sine information ROM (Read Only Memory) 57f and a D/A converter 57g. 
The counter 57e receives a clock of a speed nxc3x97fSYMBOL (Hz) which is outputted from the carrier generator 57d, and then outputs this clock corresponding to phase information. Herein, a code n denotes an integer xe2x89xa72 (normally, an integer xe2x89xa74), and a code fSYMBOL denotes a symbol clock.
The cosine information/sine information ROM 57f has an index based on addresses outputted from the counter 57e, and outputs amplitude value information regarding digital sine and cosine components. Amplitude value information regarding digital sine and cosine waveforms is outputted according to phase values obtained by subdividing 0 to 2xcfx80 at proper intervals.
The first frequency conversion unit 57a multiplies an Ich baseband signal outputted from the roll-off filter 51a by a digital cosine component outputted from the cosine information/sine information ROM 57f. The second frequency conversion unit 57b multiplies a Qch baseband signal outputted from the roll-off filter 51b by a digital sine component outputted from the cosine information/sine information ROM 57f. Then, Ich and Qch modulating signals respectively outputted from the first and second frequency conversion units 57a and 57b are coupled together in the hybrid unit 57c, passed through the D/A converter 57g and then outputted from the transmission unit 55.
The roll-off filters 51a and 51b and the transmission unit 55 are the same as the functions of the above analog system, and thus explanation thereof will be omitted.
In the digital system configured in the above manner, quadrature amplitude modulation is performed. In other words, in this digital system, a band for a baseband signal is raised by digital processing. Specifically, its band is raised by multiplying the baseband signal by a sampling clock having a speed faster by n times. More specifically, a baseband signal of a symbol clock speed fSYMBOL outputted from the roll-off filter 51a is multiplied by a digital cosine waveform having a speed faster by n times, and a baseband signal of a symbol clock speed fSYMBOL outputted from the roll-off filter 51b is multiplied by a digital sine waveform having a speed faster by n times.
A carrier frequency of nxc3x97fSYMBOL (Hz) is thereby obtained, and the band is directly up-converted to an RF band.
Here, if there is no problem for the use of an n multiple frequency of a symbol clock as a center frequency of a transmitted carrier, it is possible to configure the transmission system such that a baseband signal can be n multiple of a symbol clock synchronized with this baseband signal. But if there is a problem for the use of an n multiple frequency of a symbol clock, it is necessary to convert a signal outputted from the D/A converter 57g in the quadrature amplitude modulation unit 57 into yet another frequency.
The need of conversion into yet another frequency arises because in terms of a relationship between an outputted transmitted frequency and a symbol clock indicating a processing speed for a baseband signal, the transmitted frequency is not integral multiple of the symbol clock in most cases. In other words, in this method, a frequency obtained by processing a baseband signal at an n multiple speed of the same must coincide with the transmitted frequency. But since a value of the transmitted frequency is one predetermined based on system data, there is no coincidence between this value and the n multiple frequency of the baseband signal.
Furthermore, because of no RF elements which operate in frequency bands other than an existing band or because of no operation guarantees, an RF circuit must be configured by using one of a currently used frequency band.
On the other hand, the frequency conversion unit may obtain a transmission speed by increasing the number of modulation multivalues for a baseband signal and using an analog system. But this method is not so efficient. It is because if the number of modulation multivalues is increased by QAM or the like, the increased number of multivalues will lead to an increase in the number of digital processing bits and thus an increase in the size of a digital circuit. Consequently, cost effectiveness and power consumption will become problems. For solving these problems, it is now desired that a digital circuit should be miniaturized by devising a circuit.
Employed therefore is a method for performing frequency conversion between an outputted frequency requested by a radio system to be operated and a symbol clock frequency. In other words, a symbol clock frequency is converted into a second intermediate frequency by using a second carrier frequency, and then it is converted into a desired transmitted frequency.
An example of conversion into such a second intermediate frequency can be provided by a circuit configuration shown in FIG. 18. Shown in FIG. 18 is an example specified for the case of n=4.
Referring now to FIG. 18, shown are main sections of the transmitter of a digital modulation radio transmitter-receiver using a selector. A digital modulation apparatus 59 shown in FIG. 18 performs quadrature amplitude modulation for inputted data, converts its output into a second intermediate frequency and then converts it into a desired transmitted frequency. The digital modulation apparatus 59 comprises roll-off filters 51a and 51b operated based on symbol clocks, a transmission unit 55, a quadrature amplitude modulation unit 60 and a third frequency conversion unit 61.
Herein, the quadrature amplitude modulation unit 60 selects Ich and Qch signals by the selector and then outputs these signals. The quadrature amplitude modulation unit 60 is constructed to include phase inversion units 60a and 60b, a selector 60c, a carrier generator 60d, a 4-ary counter 60e and a D/A converter 60f. 
The third frequency conversion unit 61 converts a signal outputted from the quadrature amplitude modulation unit 60 into a desired transmitted frequency. The third frequency conversion unit 61 is constructed to include a carrier generator 61a and a mixer 61b. 
In the digital modulation apparatus 59 configured in the above manner, baseband signals from the roll-off filters 51a and 51b are inputted to the selector 60c together with signals inverted in the phase inversion units 60a and 60b of the quadrature amplitude modulation unit 60. Then, in the selector 60c, outputs are received from the 4-ary counter 60e, the outputs having been provided from the carrier generator 60d at a speed of 4xc3x97fSYMBOL (Hz), and four kinds of signals, i.e., I, Q, xe2x88x92I and xe2x88x92Q, are selected and outputted. Then, in the D/A converter 60f, a signal outputted from the selector 60c is D/A-converted at a speed of 4xc3x97fSYMBOL (Hz). Then, in the third frequency conversion unit 61, the signal is converted into a second carrier and then transmitted from the transmission unit 55.
Thus, a modulating signal is up-converted into a desired frequency by using a second carrier and then transmitted. In addition, as disclosed in Japanese Patent Laid-Open (Kokai) No. HEI 10-023096, there is available a technology for miniaturizing a digital circuit by operating roll-off filters based on symbol clocks, which is realized by circuit devising.
According to Japanese Patent Laid-Open (Kokai) No. HEI 10-023096 referenced herein, particularly disclosed regarding a QAM system digital modem is a technology for a digital modem which is used for a multiplex radio device or a CATV. Specifically, this technology has the following five aims: first, reductions in the circuit sizes of roll-off filters and in power consumption by the roll-off filters; second, prevention of an increase in the circuit size even if a carrier has a high multiplication of a symbol rate; third, elimination of the necessity of changing an oscillator frequency used for frequency conversion even if a carrier frequency is changed; fourth, prevention of cost increases for an AGC circuit even if a carrier frequency has a high multiplication of a symbol rate; and, fifth, correction of a frequency characteristic of a modulated wave after D/A conversion by a D/A converter to be flat in shape.
Referring now to FIG. 19, illustrated is an aspect of the transmitter of a digital modulation system radio transmitter-receiver disclosed in Japanese Patent Laid-Open (Kokai) No. HEI 10-023096. A digital modulation apparatus 100 shown in FIG. 19 comprises four roll-off filters 101, 102, 103 and 104, two inverting means 105 and 106, selecting and outputting means 107 and a D/A converting means 108.
Herein, the operating speeds of the four roll-off filters are not n multiple oversampling speeds but symbol clock speeds. The selecting and outputting means 107 switches signals outputted from the four elements including the roll-off filters 101 and 103 and the inverting means 105 and 106 by a sampling clock having a speed faster by 4 times than a symbol clock. The D/A converting means 108 converts and outputs a signal outputted from the selecting and outputting means 107 by a sampling clock having a speed faster by 4 times than a symbol speed.
With the configuration made in the above manner, reductions can be made in the circuit sizes of the roll-off filters of the digital modulation apparatus 100 and power consumption in the roll-off filters.
To summarize, the problems inherent in the foregoing systems provided in the related art are as follows. In the case of using the digital system shown in FIG. 17, since a transmitted frequency is a value predetermined based on system data as described above, a phase rotational speed for a baseband signal must be matched with this value. Moreover, because of no RF elements which operate in frequency bands other than an existing band or because of no operation guarantees, an RF circuit must be configured by using one of a currently used frequency band.
Another problem concerns the method for obtaining a transmission speed for a baseband signal by using a multivalued modulation system such as a QAM. The increased number of multivalues leads to an increase in the number of digital processing bits and thus an increase in the size of the digital circuit. Consequently, costs and power consumption are increased.
For solving the above problems, a digital system like that shown in FIG. 18 is utilized. In this case, transmission is performed by up-converting a modulated wave into a desired frequency by using a second carrier.
For the purpose of solving the problem of the increases in costs and power consumption among others described above, the technology for reducing a circuit size and power consumption is disclosed in Japanese Patent Laid-Open (Kokai) No. HEI 10-023096.
However, the circuit based on the above technology is complex in configuration, and there is a portion of an analog D/A converter. Thus, it is difficult to turn the circuit into LSI.
The present invention is made in order to solve the problems discussed above. It is an object of the invention to provide an apparatus which can perform frequency conversion into a transmitted frequency coincident with system data without using an analog circuit for conversion into a second frequency, increase a limit value of this transmitted frequency, simplify and miniaturize a circuit configuration by fully digitizing a circuit so as to turn the circuit into LSI and thereby increase accuracy and cost effectiveness.
To achieve the foregoing object, according to an aspect of the present invention, a digital modulation apparatus comprises a quadrature amplitude modulation unit for modulating first and second baseband digital signals orthogonal to each other by means of digital cosine/sine information obtained based on information regarding a carrier frequency which is n multiple (n is an integer xe2x89xa72) of a local frequency, and a phase rotation unit provided in the input side of the quadrature amplitude modulation unit for performing phase rotation for the first and second baseband digital signals by using a difference frequency equivalent to a difference between the local frequency and a baseband frequency.
The digital modulation apparatus thus configured is advantageous for promoting general applicability of transmitting members, since a frequency can be converted into a transmitted frequency coincident with system data without using an analog circuit for conversion into a second frequency. It is also advantageous for facilitating the turning of a circuit into LSI, since a circuit configuration can be simplified and miniaturized by fully digitizing the circuit. It is yet further advantageous for increasing circuit cost effectiveness, since the difference frequency is moved at a low speed.
The quadrature amplitude modulation unit may be constructed to include a carrier frequency/phase generator for outputting phase information by using information regarding the carrier frequency, a first cosine/sine information storage unit for receiving the phase information from the carrier frequency/phase generator and outputting digital cosine/sine information equivalent to the phase information, and an operation unit for multiplying the digital cosine information from the first cosine/sine information storage unit by the first baseband digital signal, multiplying the digital sine information from the first cosine/sine information storage unit by the second baseband digital signal and then adding together both of these multiplying results. The carrier frequency/phase generator can be constructed to include a frequency generator for generating the carrier frequency and a counter for receiving the output of the frequency generator and outputting the phase information.
Thus, since the carrier frequency/phase generator may be constructed to include the frequency generator for generating the carrier frequency and the counter for receiving the output of the frequency generator and outputting the phase information and thereby a frequency can be converted into a transmitted frequency coincided with system data without using an analog circuit for conversion into a second frequency, the above construction is advantageous for increasing general applicability of transmitting members. Since a circuit can be fully digitized so as to simplify and miniaturize a circuit configuration, it is also advantageous for facilitating the turning of the circuit into LSI. Furthermore, since a difference a frequency is moved at a low speed, it is advantageous for increasing circuit cost effectiveness.
The quadrature amplitude modulation unit can be constructed to include a carrier frequency/phase generator for outputting phase information by using information regarding the carrier frequency, a phase adjustment unit for branching the first and second baseband digital signals respectively into required numbers and then performing phase rotation for the branched first and second baseband digital signals respectively, and a selector unit for sequentially selecting and outputting the first and second baseband digital signals and the phase-rotated first and second baseband digital signals from the phase adjustment unit by using the phase information from the carrier frequency/phase generator as switching information. The carrier frequency/phase generator can be constructed to include a frequency generator for generating the carrier frequency and a counter for receiving the output of the frequency generator and outputting the phase information.
Thus, since the carrier frequency/phase generator may be constructed to include the frequency generator for generating the carrier frequency and the counter for receiving the output of the frequency generator and outputting the phase information and thereby a frequency can be converted into a transmitted frequency coincided with system data without using an analog circuit for conversion into a second frequency, the above construction is advantageous for increasing general applicability of transmitting members. Since the simple selector enables a modulation unit to be realized and circuit full digitization enables a circuit configuration to be simplified and miniaturized, it is also advantageous for facilitating the turning of the circuit into LSI can be facilitated. Moreover, since a difference frequency is moved at a low speed, it is advantageous for increasing circuit cost effectiveness.
The phase rotation unit can be constructed to include a difference frequency/phase generator for outputting phase information by using information regarding the difference frequency, a second cosine/sine information storage unit for receiving the phase information from the difference frequency/phase generator and then outputting digital cosine/sine information equivalent to the phase information, and a phase rotation execution unit for executing phase rotation for the first and second baseband digital signals by using the digital cosine/sine information from the second cosine/sine information storage unit.
Thus, the above construction is advantageous for converting a frequency into an optional transmitted frequency.
The difference frequency/phase generator may be constructed to include a difference frequency generator for generating the difference frequency and a counter for receiving the output of the difference frequency generator and outputting the phase information.
In this case, the difference frequency/phase generator can be constructed to include difference frequency setting means for setting the difference frequency and an accumulator having a multiplication unit and a buffer unit for temporarily storing the output of the multiplication unit for multiplying information regarding the difference frequency by an output from the buffer unit in the multiplication unit and then outputting information stored in the buffer.
Thus, the above construction is advantageous for increasing circuit cost effectiveness by a low-speed movement.
On the other hand, in the input side of the phase rotation unit, an oversampling unit may be provided for performing oversampling for the first and second baseband digital signals respectively.
Herein, the oversampling unit may be constructed to include an FIR filter and a linear interpolation circuit.
Thus, the above construction is advantageous for obtaining a high sampling frequency and thereby setting a local frequency to be higher than an original limit value.
According to another aspect of the present invention, a digital modulation apparatus comprises a first quadrature amplitude modulation unit for modulating first and second baseband digital signals orthogonal to each other by means of digital cosine/sine information obtained based on information regarding a carrier frequency which is n multiple (n is an integer xe2x89xa72) of a local frequency, a second quadrature amplitude modulation unit for modulating the first and second baseband digital signals by means of digital cosine/sine information different in phase by 90xc2x0 from the digital cosine/sine information used in the first quadrature amplitude modulation unit, a difference frequency/phase generator for outputting phase information by using information regarding a difference frequency which is equivalent to a difference between the local frequency and a baseband frequency, a third cosine/sine information storage unit for receiving the phase information from the difference frequency/phase generator and outputting digital cosine/sine information equivalent to the phase information, and an operation unit for multiplying the digital cosine information from the third cosine/sine information storage unit with the output of the first quadrature amplitude modulation unit, multiplying the digital sine information from the third cosine/sine information storage unit with the output of the second quadrature amplitude modulation unit and adding together both of these multiplying results.
The digital modulation apparatus thus configured is advantageous for promoting general applicability of transmitting members, since a high sampling frequency can be obtained to set a local frequency higher than an original limit value and a frequency can be converted into a transmitted frequency coincided with system data without using an analog circuit for conversion into a second frequency. With full digitization of a circuit, a circuit configuration can be simplified and miniaturized, and thus the turning of the circuit into LSI can be facilitated. As a result, high accuracy and high cost effectiveness can be promoted. It is also advantageous for increasing cost effectiveness, since a difference frequency is moved at a low speed.
The first quadrature amplitude modulation unit can be constructed to include a carrier frequency/phase generator for outputting phase information by using information regarding the carrier frequency, a phase adjustment unit for branching the first and second baseband digital signals respectively into required numbers and performing phase rotation for the branched first and second baseband digital signals respectively, and a first selector unit for sequentially selecting and outputting the first and second baseband digital signals and the phase-rotated first and second baseband digital signals from the phase adjustment unit by using the phase information from the carrier frequency/phase generator as switching information. The second quadrature amplitude modulation unit can be constructed to share the above carrier frequency/phase generator and phase adjustment unit with the first quadrature amplitude modulation unit and include a second selector unit for sequentially selecting and outputting the first and second baseband digital signals and the phase-rotated first and second baseband digital signals from the phase adjustment unit by using the phase information from the carrier frequency/phase generator as switching information and selecting phases different by 90xc2x0 from those of the first selector unit.
Thus, the above construction is advantageous for promoting general applicability of transmitting members, since a high sampling frequency can be obtained to set a local frequency higher than an original limit value and a frequency can be converted into a transmitted frequency coincided with system data without using an analog circuit for conversion into a second frequency. Since a modulation unit can be constructed by a simple selector and a circuit can be fully digitized, a circuit configuration can be simplified and miniaturized to facilitate the turning of the circuit into LSI. As a result, high accuracy and high cost effectiveness can be promoted. It is also advantageous for increasing circuit cost effectiveness, since a difference frequency is moved at a low speed.
Herein, the carrier frequency/phase generator may be constructed to include a frequency generator for generating the carrier frequency and a counter for receiving the output of the frequency generator and outputting the phase information.
Thus, the above construction is advantageous for promoting general applicability of transmitting members, since a frequency can be converted into a transmitted frequency coincided with system data without using an analog circuit for conversion into a second frequency.
The difference frequency/phase generator may be constructed to include a frequency generator for generating the difference frequency and a counter for receiving the output of the frequency generator and outputting the phase information. Also, the difference frequency/phase generator may be constructed to include difference frequency setting means for setting the difference frequency and an accumulator having a multiplication unit and a buffer unit for temporarily storing the output of the multiplication unit for multiplying information regarding the difference frequency by an output from the buffer unit in the multiplication unit and outputting information stored in the buffer unit.
Thus, the above construction is advantageous for promoting general applicability of transmitting members, since a frequency can be converted into a transmitted frequency coincided with system data without using an analog circuit for conversion into a second frequency. Since a modulation unit can be constructed by a simple selector and a circuit can be fully digitized, a circuit configuration can be simplified and miniaturized to facilitate the turning of the circuit into LSI. As a result, high accuracy and high cost effectiveness can be promoted. It is also advantageous for increasing circuit cost effectiveness, since a difference frequency is moved at a low speed.
Furthermore, in the input sides of the first and second quadrature amplitude modulation units, oversampling units may be provided for performing oversampling for the first and second baseband digital signals respectively.
The oversampling units may be constructed to include FIR filters and linear interpolation circuits.
Thus, the above construction is advantageous for promoting general applicability of transmitting members, since a high sampling frequency can be obtained to set a local frequency higher than an original limit value and a frequency can be converted into a transmitted frequency coincided with system data without using an analog circuit for conversion into a second frequency. Full digitization of a circuit enables a circuit configuration to be simplified and miniaturized to facilitate the turning of the circuit into LSI. As a result, high accuracy and high cost effectiveness can be promoted. It is also advantageous for increasing circuit cost effectiveness, since a difference frequency is moved at a low speed.